Synthesis and crystal structure of diaqua(1,4,8,11-tetraazacyclotetradecane)zinc(II) bis(hydrogen 4-phosphonatobiphenyl-4′-carboxylato)(1,4,8,11-tetraazacyclotetradecane)zinc(II)

The coordination polyhedra of the zinc(II) ions in the complex cation and the anion of the title compound, viz. trans-ZnN4O2, are distorted octahedra. In the crystal, the hydrogen-bonding interactions between the N—H groups of the tetraamine, the acidic groups of the anion and coordinated water molecules result in formation of one-dimensional tapes running along the [1 0] direction, which are further arranged in sheets lying parallel to the (001) plane.

In the asymmetric unit of the title compound, trans-diaqua (1,4,8,11-tetraazacyclotetradecane-4 N 1 ,N 4 ,N 8 ,N 11 )zinc(II) trans-bis(hydrogen 4-phosphonatobiphenyl-4 0 -carboxylato-O) (1,4,8,11-tetraazacyclotetradecane-4 N 1 ,N 4 ,N 8 ,N 11 )zinc(II), [Zn(C 10 H 24 N 4 )(H 2 O) 2 ][Zn(C 13 H 9 O 5 P) 2 (C 10 H 24 N 4 )], both Zn atoms lie on crystallographic inversion centres and the atoms of the macrocycle in the cation are disordered over two sets of sites. In both macrocyclic units, the metal ions possess a tetragonally elongated ZnN 4 O 2 octahedral environment formed by the four secondary N atoms of the macrocyclic ligand in the equatorial plane and the two trans O atoms of the water molecules or anions in the axial positions, with the macrocyclic ligands adopting the most energetically favourable trans-III conformation. The average Zn-N bond lengths in both macrocyclic units do not differ significantly [2.112 (12) Å for the anion and 2.101 (3) Å for the cation] and are shorter than the average axial Zn-O bond lengths [2.189 (4) Å for phosphonate and 2.295 (4) Å for aqua ligands]. In the crystal, the complex cations and anions are connected via hydrogen-bonding interactions between the N-H groups of the macrocycles, the O-H groups of coordinated water molecules and the P-O-H groups of the acids as proton donors, and the O atoms of the phosphonate and carboxylate groups as acceptors, resulting in the formation of layers lying parallel to the (110) plane.

Chemical context
Metal-organic frameworks (MOFs) -crystalline coordination polymers with permanent porosity -attract much current attention due to the possibilities of their applications in different areas, including gas storage, separation, sensing, catalysis, etc. (MacGillivray & Lukehart, 2014;Kaskel, 2016). Metal complexes of the tetraaza-macrocycles, in particular cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane, C 10 H 24 N 4 , L), possessing high thermodynamic stability and kinetic inertness (Yatsimirskii & Lampeka, 1985), are popular metalcontaining building units for the construction of MOFs (Lampeka & Tsymbal, 2004;Suh & Moon, 2007;Suh et al., 2012;Stackhouse & Ma, 2018). The overwhelming majority of these materials are built up using oligocarboxylates as the bridging units (Rao et al., 2004), though linkers with other coordinating groups, in particular oligophosphonates, are also used for the construction of MOFs (Gagnon et al., 2012). At the same time, hybrid bridging molecules containing both phosphonate and carboxylate functional groups have been studied to a much lesser extent (see, for example, Heering et al., 2016b), though one can expect that the combination of different acidic donor groups in one ligand molecule could open new possibilities for the creation of MOFs with specific chemical and structural features different from those inherent for MOFs formed by pure ligand classes.

Structural commentary
The molecular structure of the title compound, I, is shown in Fig. 1 (Bosnich et al., 1965). Both metal ions possess a tetragonally elongated trans-ZnN 4 O 2 octahedral environment formed by the four secondary N atoms of the macrocyclic ligand in the equatorial plane and the two O atoms of the anions or water molecules in the axial positions ( Symmetry codes: (i) Àx þ 2; Ày; Àz þ 2; (ii) Àx þ 2; Ày; Àz þ 1.

Figure 1
The extended asymmetric unit in I, showing the coordination environment of the Zn atoms and the atom-labelling scheme (displacement ellipsoids are drawn at the 30% probability level). C-bound H atoms have been omitted for clarity. Only one of two disordered components of the Zn2 cation is shown. Dotted lines represent intra-cation hydrogenbonding interactions. [Symmetry codes: (i) Àx + 2, Ày, Àz + 2; (ii) Àx + 2, Ày, Àz + 1.] group (N1-H1) of ligand L and the O2 atom of the phosphonate fragment (Table 2).
The benzene rings in the HA 2À anion in I are tilted with respect to each other [the angle between their mean planes is 40.4 (2) ], while the uncoordinated carboxylate group is close to being coplanar with the corresponding aromatic fragment [12.3 (2) ]. This carboxylate group displays a high degree of electronic delocalization [the C23-O4 and C23-O5 bond lengths are 1.251 (8) and 1.258 (8) Å , respectively], as does part of the coordinated phosphonate group [1.503 (4) and 1.511 (4) Å for the P1-O1 and P1-O2 bond lengths, respectivey]. The protonated P-O3H bond [1.583 (4) Å ] is not involved in delocalization.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.43, last update March 2022; Groom et al., 2016) indicated that several ionic compounds including ammonium and hexaamine cobalt(III) cations (refcodes SEDDUD and SEDFEP, respectively; Heering et al., 2016a) and coordination polymers formed by zinc(II) (UNISOB and UNISUH), cadmium(II) (UNITES) and mercury(II) ions (UNIWEV; Heering et al., 2016b) have been structurally characterized to date. In the polymeric complexes, the phosphonate groups of the ligands display a 3 -5 bridging function and form twodimensional metal-oxo layers. The complexation behaviour of the carboxylate groups determines the dimensionality of the polymeric systems formed. If, like in I, they are not coordinated, the metal-oxo layers are simply decorated with ligand molecules (UNISOB and UNIWEV). At the same time, the 2 -or 3 -bridging function of the carboxylate groups results in the formation of another kind of metal-oxo layer, thus producing three-dimensional coordination polymers (UNISUH and UNITES), in which the ligand molecules act as pillars. Interestingly, the tilting of the benzene rings in the ligand in polymeric complexes is much smaller that in I and does not exceed 7 (UNITES). The hydrogen-bonded tape (C atoms in green) and sheet parallel to the (110) plane in I. H atoms at C atoms have been omitted, as has one disorder component of the macrocyclic Zn2 cation. Intra-and inter-tape hydrogen bonds are shown as dashed lines in green and blue, respectively; intramolecular N1-H1Á Á ÁO2 hydrogen bonds are not depicted.

Synthesis and crystallization
All chemicals and solvents used in this work were purchased from Sigma-Aldrich and were used without further purification. The acid H 3 A was synthesized according to a procedure described previously (Heering et al., 2016b). The complex [Zn(L)](ClO 4 ) 2 was prepared by mixing equimolar amounts of L and zinc perchlorate hexahydrate in ethanol.
For the preparation of the title compound, I, a solution of [Zn(L)](ClO 4 ) 2 (23 mg, 0.06 mmol) in water (2 ml) was added to a dimethylformamide (DMF) solution (3 ml) of H 3 A (11 mg, 0,04 mmol) containing triethylamine (0.05 ml). A white precipitate, which had formed over several days, was filtered off, washed with small amounts of dimethylformamide (DMF) and diethyl ether, and dried in air (yield: 6.7 mg, 15% based on the acid). Analysis calculated (%) for C 46 H 70 N 8 -O 12 P 2 Zn 2 : C 49.34, H 6.30, N 10.01; found: C 49.45, H 6.41, N 10.21. Single crystals of I suitable for X-ray diffraction analysis were selected from the sample resulting from the synthesis. Caution! Perchlorate salts of metal complexes are potentially explosive and should be handled with care.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms in I were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.93 (ring H atoms) and 0.97 Å (methylene H atoms), and N-H distances of 0.98 Å , with U iso (H) values of 1.2U eq of the parent atoms.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (